The present invention relates to methods and systems for obtaining low-angle circumferential optical access to the eye.
Resistance in the ocular pathway results in elevated intraocular pressure (IOP), one of the most important risk factors for the development of vision-threatening glaucomatous changes. All current glaucoma therapeutics lower IOP to prevent further neuronal death from this blinding disease.
Many of the clinically most vulnerable elements of the outflow pathway (trabecular meshwork, Canal of Schlemm, collector channels, and other distal structures) cannot be imaged using conventional medical imaging modalities. These elements cannot be imaged with routine clinical ultrasound, computed tomography (CT) or magnetic resonance imaging (MRI) because the structures are too small (submillimeter) to be resolved with these techniques. These elements cannot be imaged using standard optical microscopy and can only be imaged in a limited fashion with optical coherence tomography (OCT) from the outside of the eye because they are located behind the translucent sclera, which completely obscures standard microscopy and substantially reduces the resolution and signal-to-noise ratio of standard anterior segment OCT. These elements cannot be readily imaged from inside the eye because the index mis-match at the air-cornea interface renders this angle region optically inaccessible from outside the unaltered eye, since any light reflected from the angle region toward the cornea is totally internally reflected at the air-cornea interface. As a result, diagnostics for the outflow pathway in vivo have been limited. Moreover, surgical therapeutics for the outflow pathway in vivo have also been limited to large scale invasive techniques or restricted use of thermal lasers.
With reference to diagnostics, two techniques have been used clinically to evaluate the outflow pathway: tonography and conventional gonioscopy. Tonography involves the continuous measurement of the intraocular pressure over the course of minutes in response to a deforming weight placed on the eye. Due to challenges with both the patient interface and the technical difficulty of this test, tonography is currently rarely used in the clinical setting. In contrast, gonioscopy involves the placement of a special contact lens on the eye to directly visualize the entrance of the outflow pathway from the inside of the eye, and remains a standard component of the glaucoma exam. However, visualizing only the entrance of the outflow pathway informs one only of the patency of the entrance—to distinguish between “open” or “closed” angle glaucomas. No further information about the remainder of the outflow pathway beyond the entrance (such as the trabecular meshwork and Schlemm's canal) is obtained because conventional gonioscopy does not allow for visualization below the tissue surface. These internal structures are the postulated actual sites of outflow resistance.
To view the entire extent of the outflow pathway, a tomographic imaging technique is required, preferably to view the critical structures directly from inside the eye. Optical coherence tomography (OCT) is a non-invasive, micrometer resolution optical imaging technique that has been successfully used in medicine to produce cross-sectional in vivo images of a variety of tissues. In ophthalmology, OCT has become an accepted clinical standard technique for imaging of retinal pathology. OCT is also routinely used for imaging the anterior segment, including the irido-corneal angle in the region of the trabecular meshwork from the outside, in which it is limited to providing gross anatomical views of those structures. The shortcomings of conventional anterior segment OCT for imaging the ocular outflow pathway from outside the eye include loss of signal and resolution by imaging externally through the optically translucent corneal-scleral limbus, and the ability to only measure selected angular location (typically temporal and nasal) limiting complete circumferential analysis. Histologically, the outflow pathway has been shown to vary circumferentially, and limited sampling may not identify the pathologic areas.
To overcome these issues and to maximize the imaging capabilities of OCT for this anatomical region, light would ideally be directed through the optically clear cornea into the internal entrance of the outflow pathway and scanned circumferentially. However, due to the large refractive index change between the corneal surface and the surrounding air, the irido-corneal angle is optically isolated (as a result of total internal reflection), making optical imaging of this area challenging.
As previously described, surgical therapeutics for the ocular outflow pathway in vivo have been limited to large scale invasive techniques or restricted use of thermal lasers. Large scale invasive techniques refer to “scalpel surgeries.” In these techniques, the surgeon utilizes a blade to open the ocular outflow pathway. This can be done externally as in trabeculectomy (a punch is used to locally “punch out” the entire outflow pathway in a select region), canaloplasty (an opening is created in the outflow pathway and then a catheter is driven circumferentially around the entire pathway), among other procedures. Internal techniques are also available such as in goniotomy, in which a blade is inserted into the anterior chamber and the internal opening of the outflow pathway is sliced open. These techniques share in common blunt manual dissection of an area that is mere microns in dimension.
Thermal lasers have also been used to surgically manipulate the ocular outflow pathway. These are known as laser trabeculoplasty and use the laser to heat the entrance of the ocular outflow pathway (trabecular meshwork). Typically, the surgeon uses a single faceted mirror to see one area of the trabecular meshwork and apply the laser, the circumferential extent is treated by physically spinning the facet to access the remaining areas. This treatment is primarily superficial; involvement of deeper structures is usually a result of inadvertent thermal/biological changes from the superficial laser treatment.
In an ideal scenario, the precision of the laser would be used to therapeutically change the deeper structures of the ocular outflow pathway. In contrast to “scalpel surgery,” this would provide a minimally invasive targeted means of altering any pathologic areas of the outflow pathway.
With an optical system that allows for direct OCT viewing of the ocular outflow pathway from inside the eye to create tomographic images of this area, one would have the ability to visualize the pertinent structures. Because OCT is an optical technique, laser energy can also be delivered via the same optical system. In this way, minimally invasive, image-guided therapeutics of the ocular outflow tract become possible.
Thus, improved optical access to structures of the eye would improve diagnosis and treatment of conditions related to structures inside the eye, especially peripheral structures that have heretofore been difficult to access by direct imaging methods.